Rotate a quaternion by Euler angles input

Question

I am writing a code to control robotic arm in 3D space. The robotic arm handle the rotation by quaternion but I want user to control it by changing yaw, pitch and roll since

Answer 1

Why not build the quaternions directly?

Eigen::Quaterniond rollQuaternion(cos(0.5*roll), sin(0.5*roll), 0.0, 0.0);
Eigen::Quaterniond pitchQuaternion(cos(0.5*roll), 0.0, sin(0.5*roll), 0.0);
Eigen::Quaterniond yawQuaternion(cos(0.5*roll), 0.0, 0.0, sin(0.5*roll));
Eigen::Quaterniond finalOrientation = rollQuaternion*pitchQuaternion*yawQuaternion*current_q;